Lighting for Photorealistic Renderings

Transcription

1 11/28/2005-8:00 am - 9:30 am Room:N. Hemispheres (Salon A4) (Dolphin) Walt Disney World Swan and Dolphin Resort Orlando, Florida Alexander Bicalho - Autodesk DV11-2 If you ever created a 3D model, applied materials, and then spent hours adding lights to it, this class is for you. You will learn tips and tricks on lighting, tips on how to use "fakeosity," radiosity, and GI. We'll walk you though the different light types and when to use each of them by looking at rendering the same way a photographer looks at a photo shoot. Some knowledge of 3ds Max, Autodesk VIZ, or VIZ Render is recommended. About the Speaker: Alexander has been working at Autodesk for over 4 years as a software quality analyst, focusing on radiosity and photorealistic rendering. He has been using 3ds Max for over 10 years, doing architectural visualization and renderings. Alexander also provided consulting and training, and developed scripted tools with MAXScript and AutoLISP. He was the lead author in Mastering MAXScript & the SDK for 3D Studio Max and also has written other published titles about 3ds Max.

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3 This material contains topics that will help you understand the basics of lighting. They are designed for you to exercise some topics that may not have been mentioned in the class, but will help you solidify your understanding and remember the topics mentioned in class. Lighting for Photorealistic Rendering Lighting is one of the most critical parts of computer illustration. It s through lighting that your scenes will be able to look realistic, to display depth and to actually make the image look nice. Throughout this paper and in the course, we will use 3dsmax 7, but the concepts can be applied to other products since they re very similar and industry standard. You will start learning very simple and basic concepts getting deeper and deeper as you move further along. This class requires basic knowledge of 3dsmax or Autodesk VIZ and it will not go into details sometimes. Always use the online help and tutorials if you have questions about any of the procedures that are not covered by this document. If you re using Autodesk VIZ or other products, the default values specified may not be the same. See the default value tables later on in the document for more information. Standard Lights Standard Lights have simple parameters, for instance, multiplier and color. These lights are not real-world based and can be tough to manipulate sometimes, so it s best if you understand the limitations and advantages of them so you can use them the best way you can. The first key point about Standard Lights is that they re infinitely bright. There s no concept of units, or anything. This means that no matter how far an object is it will get the same amount of light as an object that is closer to the light. This can be either a blessing or a curse. It s up to the artist to learn how to control the light and how to setup the scene to take advantage of it. Attenuation and Decay To better understand computer lighting, you should compare it with real world lighting. In the real world, lights have one interesting property: their intensity will vary based on how far the object is from the light. In fact, the ratio is not linear, it s a square ratio. This means that an object twice farther from a light will receive 1/4 th of the light. This is not what happens with Standard Lights. By default Standard lights do not have decay. This means the intensity will be the same no matter the distance. To solve this problem, you have several different ways to specify the decay. You can specify it using an inverse ratio or an inverse square ratio, you can specify where the decay starts, or, if you prefer you can also specify a series of attenuation radius that will cause the light to falloff at a certain distance. Decay will reduce the intensity of the light based on the distance. This means that if a light has been set to Inverse decay, the intensity will reduce proportionally to the distance. Inverse Square will reduce as in the real world, but if you use Inverse Square lights in your scenes, they may become too dark. This is because the computer does not have the same adaptability as the human eye, and to solve this problem you will use Exposure Control, which will be discussed in detail further. Attenuation provides a mid point between both worlds. You can manually specify where the light starts, where it has 100% intensity and where it ceases to affect the scene. Besides being able to limit the effect of a light you can also choose which objects the light affects or not. You can either include or exclude objects for each light in the scene. 2

4 Here you have 3 examples of these effects: Initial model Lights with Inverse Decay Lights with Attenuation Let s recap so far what you ve learned: - Lights in the computer have an infinite effect unless you tell them to behave differently - Attenuation and Decay can limit the light intensity either by simulating the natural physics of light or by limiting the light effect to a certain area - Objects can be included or excluded from each light Shadows and Shadow parameters Besides illuminating infinitely, lights do not cast shadows by default. When you look at Shadows, there are several types to choose from. These types can be split mainly into two types: Shadow Map and Raytrace. Shadow Maps create soft shadows, but do not support transparency. Raytraced Shadows create sharp shadows, and support transparency. Both shadows have controls for Bias and 2-Sided Shadows. Map size is a shadow map parameter that defines the quality of the shadow. Shadow map works by creating a grid on the light view and looking for what is a shadow and what s not. When using small value, you re computing a faster shadow, but one which will lack definition. What value should you use? It s about choosing a value that will generate a good shadow, while not using a lot of memory. Shadow Map = 16 Shadow Map = 256 Shadow Map = 1024 Hint Shadow Map Memory consumption A shadow map takes 4*(size²) bytes of memory. So a size of 512 will require 1MB of memory. Omni and Point lights compute shadow as if they were 6 spots, so they will use 6 times more memory. 3

5 Bias is a very important component of a shadow. It is necessary mainly because of curved surfaces. Curved surfaces are made by several triangles smoothed together. For that reason, one face could cast a shadow on the other, but you do not want to see that happen because they re supposed to be smooth. This is more visible when objects have a low number of faces. The last important option is the 2-sided shadow. Shadows are only cast from surfaces that have their normals pointing towards the light. For instance, when rendering a teapot, its interior has normals pointing outward so the shadow cast is incomplete. To solve this issue you can do one of two things: you can set the material to be twosided or you can set the shadow to be two sided. Small Bias causing artifacts Problems caused by disabling 2-sided shadows As you know, 3dsmax, VIZ and VIZ Render have different defaults for lights and shadows. Here s a table explaining these defaults: Product Autodesk VIZ VIZ Render Version or later All Defaults Name N/A Design VIZ mental ray N/A Cast Shadow On On On On Shadow Type Shadow Adv Raytrace Adv Raytrace Map Raytrace Shadows Transparency Off On On On Product 3dsmax Version 5 6 or later Defaults Name N/A MAX mental ray Design VIZ Cast Shadow Off Off Off On Shadow Type Shadow Shadow Raytrace Adv Raytrace Map Map Shadow Transparency Off Off Off On 4

6 Here s a comparison between the different shadow types explaining their advantages and disadvantages: Shadow Type Advantages Disadvantages Shadow Map Fast Processing Soft Shadows Large memory consumption No support for transparency Raytrace Shadows Support for transparency Relatively fast Requires one raytrace database per light which can lead to large memory consumption Crisp shadows Requires use of supersampling to Adv Raytrace/Area Shadows Mental ray shadow map Baking Shadows Unified raytrace database leads to lower memory consumption Support for transparency and Area shadows Average processing speed Soft Shadows antialias shadows for the Sunlight Slow processing Requires use of supersampling to antialias shadows for the Sunlight Large memory consumption Transparency can be slow and require more processing time When using billboards, trees and people, you usually need to use Raytraced Shadows in order to capture their transparent shadows. This may slow down renderings, and will also force you to use the hard shadows since you cannot render using shadow maps. In 3dsmax you can specify any light as a camera view to be rendered. The idea is simple you assign a 100% self-illuminated white material to all objects, except the billboard objects, like trees and people. You then render the light view. If you re using VIZ 2006 you can use Scene States to easily swap the materials. Then you can restore the original materials, and you can set all billboard objects to be excluded from the light and to not cast shadows. Assign the rendered baked shadow as a projector map for the light and when you render the shadows will be projected automatically. Physically based lighting By now you already have figured out that lighting in the computer doesn t follow the same rules as lighting in the real world. But how do you do it when you want to simulate real world lighting, or when you want to create photorealistic images? 3dsmax has a special light type to make that task easier: Photometric lights. Photometric lights will render lights based on physical formulas and parameters, making it easier for the artist to simulate a real world condition. For that reason, Photometric lights have their intensity values in Candelas, Lumens or Lux. These are physical units that measure light. If you do not know what they are, I suggest you take a look at physics books or on the internet. Using photometric lights requires you to create a model that s also physically correct. This means that the model needs to be done in the correct units and its size needs to represent the real world size. You will see how that works. How can I easily know if the system units are wrong? Hint Do not using Generic Units. Go to Customize > Units and choose a Display Unit that better suits you, be it metric or American. Then use tools like Measure Distance to analyze how big things are, and adjust either their size, or the system units to match the real size. 5

7 If you were using standard lights, units do not matter, but since you re using a physically based light, units are essential, very important. When opening files that were saved on different system units, a warning will be displayed. Always choose Adopt Units. When you merge two files that were in different units, MAX will already rescale them to match each other. Last, when importing or linking a file, make sure you always choose to rescale this way you do not need to worry about incorrect units and wrong sizes. Looking through the photometric UI you can see it has fewer parameters than the Standard Lights. The ones we will look at are the intensity, color and distribution parameters. A Photometric light can have 4 distributions: Isotropic, Diffuse, Spot or Web. An Isotropic light means it will distribute light equally in every direction. A Diffuse light can only be used for Linear and Area. It means that the light will be distributed equally based on the surface chosen, but it s only distributed below the icon. A Spot Light works just like a non-photometric spotlight, but the Beam/Field values have slightly different meanings than the Standard Light. For the standard light, the Hotspot defines an angle where the light intensity is 100%, and the Falloff defines the angle where the light will stop. For the Photometric Spot, a Beam defines the angle where the light intensity is 50%. And the Field defines the angle where the light stops. So, in a photometric light, the only place where the intensity is 100% is the exact center of the spot. The last distribution type is Web. A web distribution is defined by a photometric file, most commonly known as an IES file. An IES file is a text file provided by lighting manufacturers that defines the light intensity of the light. Picture a sphere around the light. Now imagine if you could specify different lighting intensities for every point of this sphere. That s how an IES file works. Another parameter exposed by the Photometric Light is color. You can set color in two ways: by specifying the Color Temperature of the light, and by using a Filter Color. Different lights have different Color temperatures. For instance, an incandescent light will look more orange, whereas a fluorescent light looks white tending towards blue. Sodium lights, used in light poles in the streets, have a very orange color. This is all defined by the color temperature of the light. By default, all lights are white, but you can choose a different color temperature based on the light type, or you can enter the temperature color directly, if you know what it is. Most light manufacturers provide the Color Temperature of their lights in the catalogs. The filter color is simply the color of the light. If you re specifying a non-white color temperature, both will be mixed to generate the final light color. Last, let s look a bit at the intensity. You can specify it in three ways: Flux, Intensity and Intensity at Distance. The flux is a function of the intensity x the area of the light. For instance, a 90 spot light will have a larger flux than a 60 spot light with the same intensity. Intensity at distance is a good tool to specify the intensity when you re not worried about precision, but you need a nice and smooth render. You can use Intensity at distance when you know you need a certain light level in key points of a scene. For instance, when designing a restaurant, I know I want 150 lux on the tables. So after placing my lights, I ll choose lx at, will draw a radius that reaches my table and will then specify 150 lux. Order or download Lighting Catalogs Hint The best tool to use when using photometric lights is a lighting manufacturer s catalog. The catalog will contain all the information you need: the intensity of the light; the beam and field when it s a spot; the color temperature; and if in some cases, you can even download IES files from the manufacturer s website. Some manufacturers will even go one step ahead and will provide you with DWG or MAX files of their luminaries. 6

8 Using Daylight Daylight is one of the most complicated scenarios to render. It s also a very complicated scenario to take pictures of, and the easiest way to do a daylight render is to consider it the same way you would consider taking pictures. Daylight could be split into 2 different components the sun light and the sky light. In a day with very clear sky, all you see is sun light. The sky has a very small contribution to the light. On a partially cloudy day, you start having a bit more contribution from the sky, and the sun becomes dimmer. If the day is totally overcast, you won t see the sun at all, and the only light source you ll have is the sky. Let s discuss some facts before we start talking about Daylight. Fact 1: The sunlight is really really bright Fact 2: The sunlight is many times brighter than light bulbs Fact 3: Finding the right balance between Sunlight, Skylight and Interior lights requires some experimentation Let s add some background to these facts. Most people want to do a rendering just by adding a Daylight System, placing it, and then expecting things to look nice. They will, but then, they also want to see the contribution of the lamps, they do not want the light to reflect on the floor, walls, etc. For this reason I took a couple pictures to show these facts and explain them better: Room with sunlight in a clear sky day note the lamp is on Room in an overcast day You can notice in the first picture that the interior light has little or no effect on the lighting. This is because the intensity of the light bulb is less than 10% of the intensity of the sun. This is a photograph, but the same would have happened with a rendered image when using Photometric Lights and Sunlight. It s important to define clearly what you want to do when you use daylight do you want to do a nice render or do you want to do a photorealistic render? It s very hard to do both, unless you think very well of the composition, exposure, and setup lights correctly. The easiest way to create a quick daylight render is by using Partly Cloudy Sky and then reducing the Sun intensity if needed to get some more contrast against the artificial lights. Another option is to do like the real world: add curtains, blinds, or reduce the transparency of glasses. Exposure Control Realistic rendering has similar requirements as photography. Since the renderings happen simulating the lighting levels in the real world, you have to treat the cameras similarly to the real world. This means you need to adjust the camera exposure based on the lighting level of the scene. 7

9 As an example, try rendering an image without using exposure control. Save the Non-Clamped color channel by specifying an RPF file output. This will allow you to see how the light is really being rendered. Once the image is rendered, right-click the VFB to see the color information for the pixel clicked. In many cases some of the brightest spots have color values way above 1.0, where some points are almost black. That broad difference is what requires you to make use of exposure controls. Exposure control Advantages Disadvantages Logarithmic Fast Processing Consistent look Can be used for animations Specular highlights are reduced Low contrast level Automatic / Linear Nice image look Requires a preprocess before the image is rendered Generates a different histogram per frame Cannot be used for animation An alternative to exposure control is to render using HDR or Floating point TIFF files. This generates a file that contains the pure rendered image and you can then process the exposure at a 3 rd party application. Indirect Illumination So far all that was discussed is direct lighting. The light comes from the lamp, hits the objects and stops there. But in the real world, objects absorb some of the light and reflect the rest. You will see some ways to simulate that reflection in the computer. Fakeosity The easiest way to add indirect light to your scene is by adding extra lights which will account for the effect of indirect lighting. This is a process some call fakeosity, since you re in fact faking the indirect light. To achieve fakeosity there are several processes. In essence all you need to do is add several lights using attenuation. These lights will be simulating the light that reflects on objects in the scene. You can include or exclude objects to the lights and also include/exclude objects from casting shadows too. Hint Non-uniform scaling lights can create cool effects Use Non Uniform Scale for Lights with Attenuation. This generates light cones that are elliptical, and allows you to really target effects where you need to. Lights also do not need to be where they are in real life. It s all about getting the effect you need. Direct Lights only Wireframe of the scene Using Fakeosity to simulate the light bouncing on the chairs 8

10 Radiosity process Radiosity is a process where the mesh is subdivided into small triangles. The direct light is distributed to all faces and then, based on each faces material properties, the light is bounced back in the environment, creating the nice looking indirect light. Combining Radiosity with Photometric Lights allows you to produce photo-realistic renderings. You can also compute results for lighting analysis. Radiosity can be used with Standard Lights, but it s not advisable. Radiosity will always be photometric, and Standard Lights need to be converted in order to be used. This will create an unpredictable result if rendering using Render Direct Illumination or Regather. Since Radiosity is based on the mesh you need to worry about how the scene is modeled. Light leaks or dark areas will appear whenever a face starts in an area that is lit and ends in an area that is not. This means that you need to be extra careful modeling floors, ceilings and anything that goes over a wall. Here are some images showing light leaks on a model, and details of how the meshing was setup to explain the light leaks: Light leaks on the floor, ceiling and Ceiling mesh problems Floor mesh problems There are two solutions for light leaks. The first one is to model correctly, making sure one face starts where another ends. This maybe hard especially when adding decoration items like paintings, molding, baseboards, etc. The second solution is to render using Regather. This will take longer, especially when rendering larger images. Again, it s all about the balance. You ll need to spend time either refining the model, or you can render using regather. There are some important things to keep in mind when using Radiosity. All the Photometric Light rules apply: use the right units, always add an exposure control. For easier control and flexibility, you should always start by adding meshing to objects either using the Subdivide Modifier or by using the Object Properties. Large backdrop objects like a ground or sky should either be excluded from radiosity or should be excluded from meshing to save memory and speed up the processing. If you want to simulate a sky light, don t use a sky object and instead just add a skylight to the scene using the sky as a texture map. Radiosity supports image based lighting through LDR or HDR images applied to skylight objects. Materials Since Radiosity is computing the light bounced on the objects this also means radiosity will be dependant on the materials of those objects. Sometimes an object color maybe too vibrant and can cause an excessive contribution to the lighting of the scene. In those cases you can opt to either adjust the colors themselves (this includes texture maps), or you can simply change the material parameters and reduce the Color Bleed value. The Architectural Material has this 9

11 property built in. Other materials require you to use a material called Advanced Lighting Override (commonly referred to as ALO in some forums). Another interesting material property is the ability to specify Self-Illuminating materials that will in fact generate light in the scene. Both the Architectural Material and the ALO have a Luminance property specified in cd/m². Use the Measure Tool to compute the Luminance of the object Hint Sometimes it s hard to figure out what value should be used as the Luminance of an object. If you want a given object to shine as a 1200cd light, all you need to do is compute its area in m². Then you can compute the luminance. To do so, in Customize > Units Setup set your Display Units to meters. Select the Object and go to the Utilities Panel. There you Select Measure. The utility will show you the area of the object in m². Divide intensity you want (1200 in this example) by the area of the object and you will have the Luminance value you need. Adaptive Meshing The latest version of VIZ brought Adaptive Meshing to Radiosity. This is a process similar to the algorithm used in Lightscape, but it only happens for Direct Lights. Adaptive Meshing will subdivide the mesh only where necessary. It adds detailed shadows and allows you to save memory and processing time, while increasing the quality of your solution. Since the Adaptive Meshing process only happens for Direct Lights, the best way to evaluate it is to set Initial Quality to 0. This will only process direct lights and will display it in the viewport, allowing you to quickly evaluate the results. Just like regular meshing, setting up object specific meshing values is also recommended. Even though you re using Adaptive Meshing, it s still recommended to add a Subdivide Modifier. The subdivide modifier has 2 purposes: it meshes to the size specified and it also generates almost equilateral triangles. When using Adaptive meshing you do not need small triangles because the triangulation will happen adaptively, but you do want to start from quasi-equilateral triangles. If you start with long skinny triangles, the end result will be poor and there maybe artifacts. Use the Subdivide Modifier prior to Adaptive Subdivision Hint Subdivide the mesh using the same size as the Initial Size. This will guarantee that your mesh will be in the correct shape and size, and any extra subdivision will happen afterwards. This way you save processing time computing meshing and you ensure your mesh is in the right shape for subdivision. 10

12 Long and skinny triangles generate poor Adaptive Subdivision Pre-subdivided mesh generates optimal subdivision Regathering x Refining the mesh There are three methods of rendering a radiosity solution. You can choose to render the direct light portion using either the Scanline Renderer or the direct light already stored in the radiosity engine. If you use the Scanline Renderer you get a nice looking direct light with nice shadows, but it takes longer to render each image. If you choose to render the radiosity mesh you may not have such a nice looking shadow, but rendering will be much faster. The third process is named Regather. Regather is a process that refines the lighting for each pixel that s being rendered. It uses an algorithm called Path Tracing. For each pixel random rays are shot and the lighting level is computed based on the average of those rays. The more rays, the better quality, and the longer it takes. Regather also always uses the Scanline Renderer lighting. Enable Store Direct Illumination to avoid Rendering Direct lights Hint Regathering always renders using Render Direct Lights. To avoid rendering the lights and shadows using that process (which can be costly), simply enable Store Direct Light for the lights before processing radiosity. You can find this option in the Object Properties of the lights. The best way to choose which process to use is to evaluate how many images you need to render from the same model. If all you need is one image, you can then simply use Regather and you re finished. If you need to render more than one image, or if you need to render an animation, then spending more time on a refined radiosity solution will save you rendering time, since all you need to do is to re-render re-using the radiosity and the renders will be super fast compared to the regathered ones. The last tool to reduce artifacts is Filtering. With VIZ 2006 you can add filtering separately for the direct and indirect portions of your lights. Filtering averages the light levels of the n consecutive vertices, where n is the filtering level. So if you have filtering = 3, this means that the lighting of the current vertex is an average of 3 vertices connected to it in all directions. If you re using a meshing value or 4 inches, this means that the lighting levels are being averaged over 24 inches. That could be a lot, and could also mean you re loosing precision. Please always add filtering with caution, and observe the results you get each step further. Regather also has Filtering, but instead of vertices, the filtering happens per pixel. The same rules apply, though. If your scene has detail but you re filtering with high values, you ll lose the detail. The larger the regather filter, the more RAM you need to process the image. 11

13 Radiosity Do s and Don ts Do: Add Exposure Control Use Photometric Lights Use Object Properties to specify different Meshing values Use Subdivide Modifier with Adaptive Subdivision Reduce Color Bleed if necessary Use Standard Lights excluded from the Radiosity solution to add effects Don t: Use Standard Lights Use small mesh values for large backdrop objects Use Large Filtering values Global Illumination Global Illumination is a different process to achieve the same results as Radiosity. In 3dsmax 7 and VIZ 2006 the results you will get using the Radiosity engine and Global Illumination are almost identical. This means you can use Photometric Lights just as you would with radiosity. The main differences are the fact that mental ray does not support the Advanced Lighting material, Skylight and does not allow you to setup an object that emits light through the ALO or the Architectural Material. The workflow is quite simple. Once you enable GI, you can specify the number of photons per light and the number of light bounces. The secret for rendering is finding a balance between the number of photons you need and the quality of the scene. When rendering photons, mental ray computes a default radius value. That value is used to smooth the photon samples. Unless you can see circle and splotches in your scene, you should not worry about the radius. 3dsmax 6 allows you to specify a minimum and maximum radius, whereas 3dsmax 7 only allows you to specify a maximum radius. Again, the same advice applies: start small and increase the numbers until you reach a satisfactory result. 100 Photons/light Render time: 8s 5000 Photons/Light Render time: 22s Photons/Light Render time: 79s When adding more photons you may also need to tweak the radius. This is required to reduce the noise. Another alternative is to use Final Gather to filter the noise. Global Illumination is a scene dependant process, which means that if you need to render a second view you will not need to recompute it. Mental ray allows you to save a GI solution to disk. 12

14 Final Gathering Final Gathering is a process just like regather it computes the lighting level per pixel at render time. It can be used to reduce the noise in a GI solution, or also to compute a quick GI solution at render time. Final Gathering is also the only way to render skylight in mental ray. It is a semi-view dependant solution - this means that if the camera moves in one animation, only the new part will be computed, while the remaining will be cached. Full Final Gather Render Time: 334s Final Gather Preview Render time: 196s To refine the GI solution, use Final Gather with Max Depth set to 1 and Max Bounce set to 0. This will make it so that final gather does not compute light bounces and it only refines your GI solution. Other methods There are many 3 rd party renderers in the market. Each one uses a different algorithm or presents that algorithm with a different UI. Some will make use of all the materials and lights available, while others will require you to use their own materials and lights. The concepts are always the same you need lights, objects, the materials will define the color that is bounced, etc. Lights can be artificial or realistic, can have decay or not, can be specified in real or empirical intensities. All you need to do is experiment. Tweak parameters slowly to increase the quality and evaluate if the results you re getting are worth the investment in time. Conclusion There are always a hundred ways to do something, what matters is what(which) way(s) you are comfortable with, and what makes you more productive. You have to try, experiment, make mistakes, and with those you ll learn something new. Another way to learn if by visiting online forums, newsgroups, user group meetings. There are several online forums dedicated to 3dsmax, VIZ, and other tools. Some forums are also dedicated to specific markets and will give you an opportunity to exchange ideas with other professionals who can help you and can also learn from your experience. 13